1. Field of the Invention
The present invention relates to dot matrix printheads and more particularly to actuators for such printheads. Still more particularly, this invention relates to actuators for wire matrix printheads in which a plurality of actuators are carried within a body and are employed to drive print wires which extend from the body into contact with a printing medium.
2. Description of the Prior Art
Printers employing wire matrix printheads are characterized in that for each print cycle, the printer does not print an entire character per impact, but instead uses an array of wire styli to print selected combinations of dots serially onto the recording medium so that as the printhead is moved relative to the medium, successive print cycles generate characters. Printheads of this type typically use a separate electromagnetic actuator for each wire stylus within the printhead.
Clapper-type matrix printheads generally include a body containing a plurality of actuators and a guide assembly which supports the wire stylii. Each actuator carried within the body includes a magnetic yoke assembly having a coil wrapped around it and an armature assembly which is movable with respect to the yoke assembly. The armature has a free end which is coupled to a wire stylus. The coil is driven so as to actuate the armature assembly in order to drive its associated stylus to impact a printing medium. A printhead of this type is disclosed in U.S. Pat. No. 4,320,981 to Harrison et al. Other dot matrix actuators are disclosed in U.S. Pat. No. 4,242,004 to Adler, U.S. Pat. No. 4,109,776 to Ek et al. and U.S. Pat. No. 3,968,867 to Stenude. Other types of electromagnetic actuators are disclosed in U.S. Pat. No. 2,998,553 to Moon et al., U.S. Pat. No. 1,998,810 to Getchell and U.S. Pat. No. 3,609,609 to Bertazzi.
Prior art actuators have various problems associated with them, including high inertia, low acceleration, low magnetic efficiency, and high energy consumption. A major factor in the design limitations of actuators is that the armature must serve the dual purpose of carrying sufficient magnetic flux to enable a large magnetic drive force to be achieved yet being rigid and light enough to cope with the stress of the impact and facilitate maximum acceleration.